Many methods for displaying color information on display devices are known in the art. Most computer systems utilize RGB (red green blue) techniques wherein color information is processed as three separate digital units of color information for each displayed pixel. For example, in a typical 24 bit RGB computer system, 8 bits describe the intensity or brightness of a red color gun of a display, 8 bits describe the intensity of a green color gun of the display, and 8 bits describe the intensity of a blue color gun of the display for a total of over 16 million possible colors for each displayed pixel.
In image processing, frequently an image will be too dark, too red, too green or have some other defect which will require an image processing operation to be uniformly applied over the entire image. For example, a user such as an artist might want to increase the red of a portrait to enhance a skin tone. The user may be able to accomplish this using a technique grayscale adjustment.
Grayscale adjustment is a technique wherein the intensity or brightness of each of the three color components of pixels may be adjusted to modify the brightness, color and/or contrast of an image (including a graphics image, a still photograph, video or the like). One of the simplest and best known techniques is shown in FIGS. 1A-1C. A display may have an input to output relationship for each color, hereinafter referred to as a grayscale, as shown in FIG. 1A. Of course, other images, displays or other types of devices such as printers or plotters may have other possible grayscales. A simple brightness control may increase the overall brightness of all colors simultaneously as shown in FIG. 1B. Note, however, that greater than 100 percent or white output is often not possible and increasing the brightness of the overall image results in brightness saturation for a larger number of input values. This saturation is shown in the flat line 10 at the 100 percent or white output value in FIG. 1B. However, if greater than 100 percent or white output is possible for a given device, then the result would be the dashed lines shown in FIG. 1B. Many systems also allow for the brightness control of each color to be modified separately. The contrast of the image may also be modified as shown in FIG. 1C. The contrast control modifies the slope of the grayscale relationship to achieve the desirable contrast.
Several software tools exist for selectively adjusting the grayscale by selectively adjusting points on a grayscale spline. A spline is a set of points that are interconnected by some form of interpolation such as straight line, Bezier or parametric techniques. For example, Picture Publisher provides 11 points that can be adjusted for each color with straight lines linking the 11 points. FIG. 2 illustrates a grayscale spline for the color red with the 11 points that can be adjusted. The user may move any one of the 11 points for this color and the system will then draw straight lines linking the moved point with the adjacent nonmoving points. This is shown in FIG. 2 as dashed lines as the 40 percent input point is set to 35 percent output. The user may also move any of the other 10 points individually to provide the desired grayscale spline. The same process may also be performed for the color blue and the color green. As a result of the modifications to the grayscale spline, an image rendered using the grayscale spline will reflect the changes selected by the user.
Another example is Photoshop which initially provides a straight line grayscale spline for each color. As shown in FIG. 3, the user may select a point on the blue spline, from the 42 percent input level in the illustrated case, and move it, to the 23 percent level output level in this case. The system then redraws the blue grayscale spline using a Bezier curve, shown as a dashed line in FIG. 3. The user may select and move another point on the blue grayscale spline which is then redrawn again as a Bezier curve through each of the selected points. This process continues one point at a time until the desired grayscale spline for the blue color is achieved. The same process may also be performed for the color red and the color green.
One problem with the above prior art techniques is that the user needs to have a detailed understanding of the relationships between color intensities and colors and the impact of changes of a grayscale spline on a final image. Otherwise, the user will probably have to perform a large amount of experimentation in order to obtain the desired effects on an image.